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Phrase(s): Butter wouldn’t melt (in someone’s mouth).
Prov. Someone is acting as if innocent. • By the time her parents came home, Emily had cleaned up all evidence of having broken the valuable figurine, and she looked as though butter wouldn’t melt in her mouth. • Jane: How can you suspect George of playing that practical joke on you? He looks so innocent. Jill: Yes, butter wouldn’t melt, I’m sure.
McGrawhill's American Idioms And Phrasal Verbs